release(csharp): MetaObjects 0.15.6 — cross-package reference resolution (ADR-0041) (NuGet)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ew1XfYSbEAezxjs9opynAe

Author: dmealing

server/csharp/Directory.Build.props

@@ -8,7 +8,7 @@
   -->
 
   <PropertyGroup>
-    <Version>0.15.5</Version>
+    <Version>0.15.6</Version>
     <Authors>Doug Mealing</Authors>
     <Company>Doug Mealing LLC</Company>
     <Product>MetaObjects</Product>